1. Field of the Invention
The present invention relates to a rigid, optically transparent heat and impact resistant polyurethane.
2. Background of the Invention
Currently, the standard material to which all optically transparent plastic materials are compared to for impact resistance is polycarbonate. These materials can be characterized by impact resistance and the temperature and pressure at which the material undergoes distortion. The heat distortion temperature of polycarbonate is about 280.degree. F. at 264 psi fiber stress.
Polycarbonate extruded sheet at a thickness of 0.25 inches has a 0.22 caliber FSP (fragment simulating projectile) V-50 impact rating of 925 ft./sec. The V-50 is the measure of the velocity at which 50% of 22-caliber projectiles fired through a 0.25 inch polycarbonate sheet penetrate a 20 millimeter-thick 2024 T6 aluminum sheet (the "witness sheet") placed 6 inches behind the back surface of the polycarbonate sheet.
U.S. Pat. No. 3,866,242, which is incorporated herein by reference, discloses a polyurethane polymer protective shield. The polyurethane is produced by:
(a) reacting either a polyether glycol or a polyester glycol having a molecular weight of from about 700 to 1,000 with methylenebis(cyclohexyl isocyanate) in an equivalent ratio of about three NCO to each hydroxyl to form a prepolymer, and
(b) reacting the prepolymer with an aromatic amine curing agent having a methylene bridge between two aromatic rings, such as 4,4'-methylenebis(2-chloroaniline), in an equivalent ratio of 0.90 to 1.04 NH.sub.2 /1.0 NCO.
U.S. Pat. No. 4,808,690, which is incorporated by reference herein, discloses a transparent polyurethane polymer made from a polyol cured prepolymer. The prepolymer is made from a polyisocyanate and at least one multifunctional hydroxy-containing intermediate.
U.S. Pat. No. 4,208,507 discloses a flexible polyurethane-urea elastomer prepared by reacting: (A) a prepolymer obtained by reacting an essentially difunctional polyhydroxy compound having a molecular weight of from 600 to 10,000, and an organic diisocyanate having at least one NCO group bonded to a cycloaliphatic structure, in amounts which provide a total OH:NCO ratio of from 1:1.2 to 1:10, with (B) 3,3', 5,5'-tetramethyl-4,4'-diamino-diphenylmethane, A and B being reacted in a molar ratio of from about 1:0.8 to 1:1.2.